1. Field of the Invention
The present invention relates generally to an apparatus and method for transmitting channel information in a mobile communication system, and in particular, to an apparatus and method for transmitting reverse channel information in a mobile communication system.
2. Description of the Related Art
In general, a mobile communication system provides service to a plurality of mobile stations using a scheduling method. The service includes voice and data communication service. The term “scheduling” refers to a process of granting a right to provide a service to a particular mobile station among a plurality of mobile stations according to their priorities or other factors, and determining data rates of services provided to the mobile stations. Because the services are divided into voice and data, the scheduling is performed in different ways for each. For voice service, the scheduling needs only the information on channel and power resources available for the voice service.
However, for data communication or service, the scheduling needs more information because data generally consumes more channel and power resources compared with the voice service. Therefore, to provide data service, the channel state, location, and the priority of a particular mobile station, etc. should be considered.
The data service can be divided into forward data transmission and reverse data transmission according to its transmission direction. The “forward data transmission” refers to data transmission from a base station to a mobile station, and the “reverse data transmission” refers to data transmission from a mobile station to a base station. The forward data transmission and the reverse data transmission need different scheduling information. A description will now be made of information necessary for the forward data transmission and information necessary for the reverse data transmission.
Data transmission in a radio link is achieved through a packet data channel on a per-physical layer packet (PLP) basis. In a system with scheduling, a mobile station gets permission, or a grant, from a base station to transmit packets in the reverse direction. In a particular system, a mobile station may occasionally start reverse data transmission at a lowest possible data rate. However, when the data rate is increased or decreased after the initial transmission, the mobile station can transmit reverse data in response to a grant from a base station, i.e., in response to a control signal from the base station Therefore, the foregoing case in which initial transmission is possible at the lowest data rate will not be considered herein. As described above, when mobile stations want to transmit reverse data, a base station performs scheduling on the mobile stations on a per-transmission time basis to grant or deny reverse packet data transmission. The scheduling information is transmitted to mobile stations by the base station. As a result, only the mobile station receiving the reverse transmission grant from the base station transmits a reverse packet. Although a physical channel over which the mobile station transmits packet data in response to the reverse transmission grant can have different channel names according to systems, it is generally called a “reverse packet data channel (R-PDCH).”
As described above, the base station performs scheduling collectively considering channel conditions and mobile station conditions in a process of granting/disapproving reverse data transmission of mobile stations. Therefore, a scheduler of the base station should have a variety of state information for the mobile stations. For example, the state information includes information on the amount of transmission data stored in a buffer of each mobile station and a reverse channel for each mobile station. Therefore, in a common mobile communication system adopting the scheduling scheme, mobile stations should report (or feed back) the scheduling information to the base station. Although a reverse channel over which the mobile stations feed back the scheduling information to the base station can have different channel names according to systems, it is generally called a “reverse request channel (R-REQCH).”
Table 1 illustrates information transmitted over the R-REQCH in a CDMA2000 Release D system, a North American version of a mobile communication system, by way of example.
TABLE 1R-REQCH InformationFieldNumber of BitsRESERVED1MAXIMUM_TPR4SR_ID3EVENT4
The information transmitted over the R-REQCH and the number of bits of the corresponding information, illustrated in Table 1, are subject to change in other systems. In any case, however, in a system adopting the scheduling, mobile stations transmit the similar feedback information to a base station. A description will now be made of the information fields illustrated in Table 1.
(1) RESERVED: A currently undefined bit, and can be used for various purposes in the future.
(2) MAXIMUM_TPR: Information on the reverse channel state of a mobile station, and indicates the maximum TPR that a mobile station can use for R-PDCH. The abbreviation “TPR” stands for a traffic-to-pilot ratio, and refers to a ratio of transmission power of an R-PDCH to transmission power of a pilot channel. In a general mobile communication system, power of mobile stations is controlled by a base station. A description will now be made of a general reverse power control process performed in a mobile communication system.
If a reverse channel of a mobile station is in a bad state, a base station orders the mobile station to increase power of its pilot channel. In contrast, if the reverse channel is in a good state, the base station orders the mobile station to decrease power of its pilot channel. In this way, the base station maintains a reception state of a reverse radio link. In other words, the base station maintains a received reverse signal-to-noise ratio (SNR) for each mobile station in a time-varying reverse radio link. Therefore, it can be considered that a level of pilot transmission power of a mobile station controlled at a particular time indicates a channel state of the corresponding mobile station. For example, assuming that there are a mobile station A and a mobile station B in communication with a base station, if pilot transmission power of the mobile station A is higher than pilot transmission power of the mobile station B at a specific time, it can be regarded that a radio channel of the mobile station B is superior in channel state to a radio channel of the mobile station A.
Commonly, mobile stations are limited in maximum transmission power. For example, if maximum transmission power of a mobile station is limited to 200 mW, it means that possible maximum transmission power of the mobile station at a particular time should not exceed 200 mW. In this situation where the maximum transmission power is limited, if a pilot channel of a mobile station is being power-controlled, power allocable to an R-PDCH by the mobile station at a particular time is changed according to a power level of the pilot channel. That is, it means that the power allocable to an R-PDCH is changed according to whether the pilot channel of the mobile station is in a good state or a bad state. For convenience, it will be assumed herein that physical channels transmitted by mobile stations include only two types of channels: a pilot channel and an R-PDCH. Actually, however, the physical channels transmitted by the mobile stations can include other channels as well.
On this assumption, if a mobile station is being power-controlled by the base station and power allocated to its pilot channel at a particular time is 50 mW, available power allocable to an R-PDCH becomes 200 mW-50 mW=150 mW. In this case, a ratio of maximum power allocable to the R-PDCH by the mobile station to power of the pilot channel is 150 mW/50 mW=3. MAXIMUM_TPR refers to the ratio of maximum power allocable to the R-PDCH by the mobile station to power of the pilot channel. The MAXIMUM_TPR is commonly expressed in dB. The mobile station feeds back information on its reverse channel state by transmitting the MAXIMUM_TPR, i.e., the ratio of maximum power allocable to the R-PDCH to power of the pilot channel, to the base station. Based on the MAXIMUM_TPR received from the mobile station, the base station can determine a reverse channel state of the mobile station, and can also determine a maximum data rate allocable to the mobile station in a scheduling process.
(3) SR_ID: Is a service identifier corresponding to buffer information, indicating the amount of data stored in a buffer, transmitted over an R-REQCH by a mobile station. For example, assuming that a mobile station alternately transmits a packet for a service A and a packet for a service B, if the amount of data stored in a buffer of the mobile station for the service A and the amount of data stored in a buffer for the service B at a particular time are 100 bytes and 300 bytes, respectively, then the mobile station sets the SR_ID to a value corresponding to the service A and writes “100 bytes” in a field indicating the amount of data stored in a buffer before transmission, when feeding back information on the amount of data stored in a buffer for the service A. The service identification information written in the SR_ID can be previously agreed upon with the base station.
(4) EVENT: Indicates the amount of data stored in a buffer, corresponding to the SR_ID.
As described above, in a mobile communication system, a mobile station transmits its reverse channel state information and buffer information to a base station over an R-REQCH, and the base station schedules reverse transmission using the information received from the mobile station.
Table 2 illustrates MAXIMUM_TPR field values and their associated maximum TPRs in a mobile communication system, by way of example.
TABLE 2MAXIMUM_TPR Field ValuesMAXIMUM_TPRMaximum TPR on R-PDCH [dB]0000TPR < 40001 4 ≦ TPR < 50010 5 ≦ TPR < 60011 6 ≦ TPR < 70100 7 ≦ TPR < 80101 8 ≦ TPR < 90110 9 ≦ TPR < 10011110 ≦ TPR < 11100011 ≦ TPR < 12100112 ≦ TPR < 13101013 ≦ TPR < 14101114 ≦ TPR < 16110016 ≦ TPR < 18110118 ≦ TPR < 20111020 ≦ TPR < 241111TPR ≧ 24
As can be understood from Table 2, in a CDMA2000 Release D system, MAXIMUM_TPR is expressed using 4 bits. Referring to Table 2, a mobile station transmits ‘0000’ if a TPR value available for an R-PDCH is less dB, transmits ‘0001’ than 4 dB, transmits ‘0001’ if the TPR value falls within a range between 4 dB and 5 dB, and transmits ‘0010’ if the TPR value falls within a range between 5 dB and 6 dB. In this way, the MAXIMUM_TPR values are expressed at 1-dB intervals between ‘0000’ and ‘1010’. In addition, MAXIMUM_TPR=‘1011’ indicates a range between 14 dB and 16 dB, MAXIMUM_TPR=‘1100’ indicates a range between 16 dB and 18 dB, and MAXIMUM_TPR=‘1101’ indicates a range between 18 dB and 20 dB. Furthermore, MAXIMUM_TPR=‘1110’ indicates a range between 20 dB and 24 dB, and MAXIMUM_TPR=‘1111’ indicates a TPR value larger than or equal to 24 dB. In the foregoing case where the TPR value available for an R-PDCH of a mobile station is expressed with 4 bits, TPR values smaller 14 dB are expressed at 1-dB intervals, and TPR values larger than 14 dB are expressed at 2-dB intervals. In addition, TPR values larger than 20 dB are expressed at 4-dB intervals. Next, with reference to Table 3, a description will be made of TPR values according to encoder packet (EP) sizes used for an R-PDCH in the CDMA2000 Release D system.
TABLE 3TPR ValuesEP size [bit]TPR [dB]1920.754083.757926.7515609.625309611.875463213.625616814.875924016.62512312181538419.125
Table 3 illustrates examples of TPR values used for an R-PDCH in the CDMA2000 Release D system. The phrase “TPR values used for an R-PDCH” refers to TPR values specified to be used for respective packet data rates. For example, in Table 3, a TPR value corresponding to an encoder packet size of 192 is 0.75 dB. When Table 3 is used, a mobile station transmits reverse packets using a frame having a fixed 10-ms length. As a result, encoder packets having 192 bits, 408 bits, . . . , and 15384 bits are transmitted for a 10-ms frame. Therefore, their data rats become 19.2 Kbps, 40.8 Kbps, . . . , and 1.5384 Mbps, respectively. That a TPR value for EP size=192 is 0.75 dB indicates that a TPR value, i.e., a ratio of R-PDCH power to pilot power, used for transmission of 19.2-Kbps packet data by a mobile station is 0.75 dB.
It can be understood from Table 3 that the EP size increases approximately on a double basis from 192 bits to 3096 bits, and increases approximately on a one-and-a-half-times basis at 3096 bits or more. This is because the increase in EP size on a double basis at a large-EP size range reduces a resolution of distribution of the EP sizes. For this reason, the TPR value corresponding to the EP size increases at about 3-dB intervals from 192 bits to 3096 bits, and increases at less-than-2-dB intervals at 3096 bits or more.
Table 2 and Table 3 show the opposite resolution tendencies. That is, in Table 2, TPR values available for an R-PDCH of a mobile station, feedbacked over an R-REQCH, are densely arranged at their smaller values and sparsely arranged at their larger values. In Table 3, TPR values corresponding to EP sizes are sparsely arranged at about 3-dB intervals at their smaller values and densely arranged at their larger values.
Table 4 illustrates a combination of Table 2 and Table 3, i.e., a relation between MAXIMUM_TPR field values, their associated maximum TPRs, and EP sizes.
TABLE 4Relation Between MAXIMUM TPR, Maximum TPRs and EPS SizesMaximum TPR on R-PDCHMAXIMUM_TPR[dB]EP size [bit]0000TPR < 4 192, 4080001 4 ≦ TPR < 5—0010 5 ≦ TPR < 6—0011 6 ≦ TPR < 7 7920100 7 ≦ TPR < 80101 8 ≦ TPR < 90110 9 ≦ TPR < 10 1560011110 ≦ TPR < 11100011 ≦ TPR < 12 3096100112 ≦ TPR < 13101013 ≦ TPR < 14 4632101114 ≦ TPR < 16 6168110016 ≦ TPR < 18 9240110118 ≦ TPR < 2012312, 15384111020 ≦ TPR < 24—1111TPR ≧ 24—
In Table 4, the third column represents EP sizes corresponding to TPR ranges indicated by MAXIMUM_TPR values fed back over an R-REQCH. Now, with reference to Table 4, a description will be made of TPR values and their associated EP sizes. MAXIMUM_TPR=‘1101 ’ on the R-REQCH indicates that a TRP value available for an R-PDCH by a mobile station ranges falls within a range between 18 dB to 20 dB. EP sizes falling within the foregoing range include 12312 and 15384 in Table 3. If a mobile station transmits a MAXIMUM_TPR value to a base station in the foregoing method, the base station preforms scheduling using the feedback information received from the mobile station. In some cases, however, the feedback information includes two or more EP sizes as described above. In this case, the base station cannot correctly determine whether an EP size supportable by the mobile station is 12312 or 15384, causing a possible failure in scheduling. The failure in scheduling reduces the entire transmission throughput of a mobile communication system.